concerning the Constitution of Matter. 197 



and evidently 



where d% ' drj d% is a volume-element in the actual condition 

 of the medium, /, g, h the displacements of the aether which 

 now fills this volume-element, and a d%' drf d% &c, the forces 

 exerted on the element of aether. Remembering (1), (3), 

 (5), and (7), we may write the last equation 



|§ = -Iff(««r+6A+^i)C^'^'; • • (W) 



=ry and ™r having similar values, and ^- &c. being simi- 

 larly obtainable. 



We may also express a, b, c in terms of the components of 



stress [ffj, [rjS] & c « ; thus 



*—§.[« -^'W-§'[W, I- • .(15) 



In this section it has been virtually assumed that the strain- 

 figure is exactly superimposed on the otherwise existing 

 condition of the medium. 



6. If the motion of the strain-figure is one of pure trans- 

 lation, a>!, co 2 , a> 3 are all constantly zero, and equation (10) 

 reduces to 



F=(11)X+(12)Y + (31)Z; 

 at the same time .. .. .. ' na ^ 



G = (12)X + (22)Y+(23)Z; 



H=(31)X + (23)Y+(33)Z; 



Now construct the ellipsoid 



(ll)r + (22V+(33)^ 2 + 2(23)^ + 2(31)?? + 2(12)^ = M6^ (17) 



and it is evident from (16) above that when the motion is 

 one of pure translation, the effective force (components F, G, 



